Maria Full of Grace and The Magdalene Sisters are both movies that I assumed would just be too depressing to watch. Both movies blew me away and made a real impact.
Big Fish is one that I figured would be just outlandishly bad. It’s one of the most beautiful movies I’ve ever seen.
Another vote for both Casablanca and Fight Club, as well as L.A. Confidential.
L.A. Confidential has slipped a little in my estimation, though, since I’ve found out that Russel Crowe wasn’t acting in that movie; he was just being himself.
And Branagh’s Henry V was also my intro to Shakespeare; we also did the “read in H.S. English class” thing, and nothing sucks the life out of Shakespeare like having it read aloud by a quasi-literate mouth breather:
"To…
…be…
…or…
…not…
…to…
…be…"
You get the idea.
So, as bad as Brangh’s Henry V might be compared to other professional works, it made a hell of a first impression, by way of comparison to having it read aloud in class by disinterested, middle-class, midwestern, American teenagers.
Sorry, I just need to dispell this idea that Brannagh’s Henry V is considered bad compared to other Shakespeare adapations. Obviously, quality in film is subjective, but few other Shakespearean films have met with the critical and commercial success that Henry V did. It’s arguably the definitive film version, and since the only other contender start Laurence fuckin’ Olivier, that’s saying quite a bit. Anyone who tells you different is probably French.
Just trying to avoid getting entangled in a “Branagh vs. Olivier” debate with Lust4Life and others. 
A family member twisted my arm to see Vanilla Sky. I think that Tom Cruise is a loathsome creep and a shi**y actor, so this film really surprised me. I can only conclude it was done right by accident…
“Full Metal Jacket”.
I went to see it with a bunch of rowdy mates, expecting to see a blood-drenched Vietnam war slaughterfest, or another “Apocalypse Now”.
Despite the fact that I hated it (or thought I hated it) when I walked out of the cinema, something about it niggled inside of me and I went back and saw it 3 or 4 more times. Not only now is it one of my very favourite films of all time, but it was my gateway to all the wonder that is Stanley Kubrick.
mm
My accidental discovery of the sheer awesomeness that is Moulin Rouge! is detailed in this thread.
Great, great movie.